Multispecimen fatigue cracking machine

ABSTRACT

This disclosure is directed to a machine for inserting fatigue cracks in up to eight specimen at once while allowing independent observation and stress control of each specimen. Eight specimen are fixed in a circular arbor and means is provided for repetitively flexing the specimen to cause cracks to initiate and grow from the root of a notch to a desired depth.

United States Patent Krafft Aug. 1, 1972 [54] MULTISPECIMEN FATIGUE CRACKING MACHINE 72 Inventor: Joseph M. 1mm, Alexandria, Va. [73] Assignee: The United States of America as represented by the Secretary of the Navy [22] Filed: March 24, 1971 211 Appl. No.: 127,623

52 us. Cl. ..73/100, 73/91 [51] Int. Cl. ..G0ln 3/32 [58] Field of Search ..73/9l, 100

[56] References Cited UNITED STATES PATENTS 2,404,584 7/1946 Liska et al ..73/ 100 X 2,706,907 4/1955 Cox ..73/9l Primary Examiner-Jerry W. Myracle At!0rney-R. S. Sciascia, Arthur L. Branning and M. L. Crane [57] A ABSTRACT This disclosure is directed to a machine for inserting fatigue cracks in up to eight specimen at once while allowing independent observation and stress control of each specimen. Eight specimen are fixed in a circular arbor and means is provided for repetitively flexing the specimen to cause cracks to initiate and grow from the root of a notch to a desired depth.

' 6 Claims, 2 Drawing Figures PAIENTEDMI m2 3.680.367

Jf x I INVENTOR JOSEPH M. KRAFFT ywzkmem ATTORNEY PATENTEDAus 1 m2 3,680,367

sum a nr 2 F/G. Z INVENTOR JOSEPH M. KRAFFT AGENT ATTORNEY MULTISPECIMEN FATIGUE CRACKING MACHINE STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor,

BACKGROUND on THE mvermou This invention is directed to fatigue cracking of notched specimen and more particularly to a multispecimen fatigue cracking machine.

The standard ASTM method of evaluating plane strain fracture thoughness K requires preliminary insertion of low stress fatigue cracks in the notch of aspecimen. in the. early efforts to characterize brittle fracture, cracks were simulated with sharply tippednotches. Eventually, it became apparent that no matter how sharp the notch was machined, a natural fatigue crack would often result in still lower values of the fracture toughness. Since preexisting cracks rather than DESCRIPTION or THE INVENTION Now referring to the drawing, there is shown for illustrative purposes in FIG. 1, a fatigue cracking machine made in accordance with the teaching of this invention. As shown, the machine includes three main assemblies, a specimen holding assembly 10, a rotata ble element 11 for depressing specimen held by the specimen holding assembly and a drive motor 12. The

motor is held in place on a plate 13, the specimen as sembly is secured in a fixed position by an upright bracket 14 which is secured to the plate 13 and the rotatable element 11 rotates about one end of the designed for one specimen at a time, are very slow and v costly. At a typical growth rate of two microinches per cycle, about 50,000 cycles are required for 1/10 inch of crack growth, typically 25 minutes at 2000 cycles per minute. it is not unusual to average 60 minutes per specimen, allowing time for stepup, crack irritation from the notch, and shut down for intermittent examinations. Aside from the processing time, the capital equipment costs of cracking with a high quality fatigue testing machine adds to the overall expense and inconvenience through tie up and wear on the precision test apparatus.

STATEMENT OF THE OBJECT S It is therefore an object of the present invention to provide a simple means for simultaneously producing a low stress fatigue crack in a plurality of specimen.

Another object is to provide a device for simultaneously producing fatigue cracks in a plurality of specimen while allowing independent observation and stress control of each specimen.

Still another object is to provide a more stablesystem for growing fatigue cracks simultaneously in .a plurality of specimen.

Yet another object is to provide a simple devicefor fatigue crack formation which may be operated by un skilled as well as by skilled personnel.

Other and more specific objects of the invention will become obvious to those skilled in the art from the following description when considered in connection with the accompanying drawings.

BRIEF DESCRIPTION-OF THE DRAWING FIG. 1 illustrates a view of the overall device. F IG. 2 illustrates a cross-section of a side view.

specimen-holder.

The specimen assembly 10 includes an arbor 16 which is machined from a solid cylindrical element 1 such as cold rolled steel. The arbor is machined to in-.

'clude eight longitudinal slotsl7, a centrally located circumferential slot 18 and a shallower circumferential slot 19 near one end. The bottom of each of the longitudinal slots include a raised ridge 21 in the area of the circumferential slot 18 which acts as a fulcrum for the specimen. The circumferential slot 18 is provided to enable one to view the specimen during fatigue cracking. The circumferential slot 19 serves to receive a retaining spring 20 which restrains the inner end of the specimen when in use. The end that includes slot 19' .is provided with an axially aligned threaded hole 22 within which a bolt 23 is threaded to hold the specimen assembly in place on the bracket 14. The opposite end is provided with a cap disk 27, which is installed by bolt 36 to keep the specimens in their slots during the fatigue cracking. The arbor is provided with a boss 24 on one end about which an idler ring 25 is matched and a spacer 29 is provided to space the specimen assembly from the upright bracket 14. I

The outer end of the arbor isfrtted with a specimen retaining ring 26 which is secured by apress or shrink fit about the end of the arbor. The ring is provided with radially extending threaded holes 28 which are spaced sufficiently to align with the centerof the slots. Bolts 31 are scr'ew'threaded into the threaded .holes 28 which apply a desired force on the end of a specimen in the slot. A nut 32 is provided on the bolts to lock the bolts I in place when the nut is threaded against the outer surface of the ring. The opposite ends of the specimen are held in place by the spring with the ends of the specimen projecting under the idlerring 25.

The idler ring is made with an outer ring portion 33 whichsurroundsthe inner end of arbor and is press fitted thereon and an inwardly extending end portion 34 that matches with the boss 24 of the arbor and tits against theinner end of the arbor. The outer ring 33 of the idler ring is provided with radially extending non- .threaded holes 30 that are centered on the longitudinal slots and flanges .35 which extend outwardly from op- 'posite sides. of the non-threaded holes and are spaced to provide a radial track therebetween. Plungers 37 are fitted within the non-threaded holes for radial motion therein and are of a length that they extendbeyond each surface of the ring portion. The plungers 37 are I each provided with a linear flat surface along the length of the central region which cooperates with a set screw 38 to prevent the plungers from falling from the hole whilev allowing linear motion. The inner end of plungers 37 and the bolts 31 are slightly pointed, for instance, a 3 degree cone, to provide symetry of loading contact on the specimen. The specimen is positioned within the arbor such that the bottom of the specimen in the area of a preformed notch 40 is over the fulcum with the bolts 31 applying a force on the outer end while the top of the inner end of the specimen is against the plunger 31.

The rotatable element 11 is rotatable about bearings on the spacer between the upright bracket 14 and the inner end of the specimen assembly. The rotatable element includes a V-pulley wheel 41 which has a greater diameter than the specimen assembly and is centered about the axis of the specimen assembly. Three roller wheels 42 spaced 120 apart are secured to the pulley wheel by suitable eccentric studs 43 with their axis on a circle about the axis of the pulley wheel. The wheels may be well known bearings which have an outer ring of a width commensurate with the spacing between the flanges 35 and the idler ring 25. The bearings are ar ranged such that they rotate on the outer surface of the idler ring and since the studs are eccentric, the bearings may be adjusted to preload the bearing against the idler ring and to center the pulley wheel on the axis of the specimen assembly. The studs are provided with nuts 44 on the opposite side of the pulley wheel which secured the studs in place and a bolt 45 with washer 46 secures the bearing on the other end of the stud. The V-pulley wheel is driven by a suitable belt 47 which loops the V-pulley wheel and a drive wheel 48'which is secured to the motor shaft.

In operation and setting up the specimen for fatigue The fatigue operation begins by starting motor 12 which rotates the V-pulley wheel. As the pulley wheel is rotated the roller bearings or wheels attached thereto rotate along the outer surface of the idler ring depressing the plungers above the inner end of each specimen as the pulley wheel is rotated. Since there are three bearings secured onto the pulley wheel each specimen will be flexed three times for each revolution of the pulley wheel.

Since the arbor is provided with a circumferential slot in the area of the notch in the specimen, the notches can be visually examined. In order to visually examine each of the specimen, the motor may be sembly to the upright bracket. Once the specimen asring. The specimen is moved to the inner end and the 1 spring is raised permitting the inner end of the specimen to be forced under the spring. The specimen is moved inwardly until the inner end is below a plunger in the idler ring. With the inner end of the specimen under the plunger, the outer end will slide into place in front of the flange on the outer ring. Also, the notch will be positioned outwardly of and above the fulcrum on the arbor. All specimen are loaded into their respective slots. With the specimen in their slots, the V-pulley wheel is rotated until the roller bearing or wheel secured thereto is directly over one of the plungers, At this time a displacement transducer (not shown forclarification of the drawings) is installed in the, outward facing notch of the specimen over which the bearing is positioned. The displacement transducer measures the sembly is rotated to a desired position, the bolt may be tightened to secure the specimen assembly in place Since the bolt is mounted on the axis of the specimen assembly no further adjustment of the specimen will be necessary.

In the event a crack length of a desired length is obtained on any one of the specimen, that specimen can be unloaded by relaxing the force applying screw which holds the outer end downwarded. Since the arbor is stationary this can be done without stopping the operation on the remaining specimens.

It has been determined that an eight inch outside diameter V-pulley wheel, driven by an two inch outside diameter drive pulley on a 850 RPM motor will result in an orbital speed of 200 RPM for the machine. Since there are three bearings that contact each specimen, each revolution of the pulley wheel, 200 RPM of the pulley wheel will. be equivalent to 600 cycles per minute or about 50,000 cycles in 80 minutes. This is equivalent to about 10 minutes per specimen. For such a machine, the arbor is 3% inches, therefore, it can be seen that eight specimen can be fatigue cracked in a relatively short time in a relatively small space. It is possible 'to drive themachine faster than this for even more rapid processing.

This machine permits an operatorto visually observe thecrack as it develops, operation time per specimen is drastically reduced and different specimen are being tion. Such an arrangement would permit immersion of the specimen in a coolant or hot bath for producing fatigue cracks under other than ambient conditions.

. Further, the device could be operated in a vacuum or with different gases surrounding the specimen to determine effects of other conditions on different specimen.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

What is claimed and desired to be secured by Letters Patent of the United States is:

1. A multispecimen fatigue cracking device, which comprises:

a specimen assembly means adapted for holding a plurality of elongated specimen,

said specimen assembly includes a fulcrum centrally claimed in claim 1; in which,

said specimen assembly means is cylindrical with linear slots therein within which said specimen are held and circumferential slots for observing and holding said specimen in place.

3. A multispecimen fatigue cracking device as claimed in claim 2; in which,

4 said means for sequentially depressing and relaxing one end of each of said specimen includes more than one depressing and relaxing element which revolves about the linear axis of said specimen assembly.

4. A multispecimen fatigue cracking device as claimed in claim 3; wherein,

said depressing and relaxing means is a rotatable wheel. 5. A multispecimen fatigue cracking device as claimed in claim 4; in which,

said specimen securing means includes a retaining ring on the outer end of said specimen assembly means and an idler ring on the inner end of said specimen assembly means,

said means for depressing and holding said specimen in a depressed position includes threaded bolts screw threaded through said retainer ring on the outer end of said specimen assembly means,

said idler ring on the inner end of said specimen having non-threaded holes therethrough on different radii thereof,

a plunger secured in each of said non-threaded holes in for movement within said hole,

said plunger positioned to contact the end of said specimen and for movement by said rotatable wheels for depressing and relaxing said specimen.

. 6. A multispecimen fatigue cracking device as claimed in claim 5; which comprises,

a lock nut on said bolts which look said bolts in place in order to secure one end of said specimen in a depressed position. 

1. A multispecimen fatigue cracking device, which comprises: a specimen assembly means adapted for holding a plurality of elongated specimen, said specimen assembly includes a fulcrum centrally of the length of each of said specimen, means for securing said specimen on said specimen assembly, means for depressing and holding one end of each of said specimen in a depressed position, and means for sequentially depressing and relaxing the opposite end of each of said specimen, whereby each of said specimen are repetitively flexed thereby causing cracks to initiate and grow in each of said specimen.
 2. A multispecimen fatigue cracking device as claimed in claim 1; in which, said specimen assembly means is cylindrical with linear slots therein within which said specimen are held and circumferential slots for observing and holding said specimen in place.
 3. A multispecimen fatigue cracking device as claimed in claim 2; in which, said means for sequentially depressing and relaxing one end of each of said specimen includes more than one depressing and relaxing element which revolves about the linear axis of said specimen assembly.
 4. A multispecimen fatigue cracking device as claimed in claim 3; wherein, said depressing and relaxing means is a rotatable wheel.
 5. A multispecimen fatigue cracking device as claimed in claim 4; in which, said specimen securing means includes a retaining ring on the outer end of said specimen assembly means and an idler ring on the inner end of said specimen assembly means, said means for depressing and holding said specimen in a depressed position includes threaded bolts screw threaded through said retainer ring on the outer end of said specimen assembly means, said idler ring on the inner end of said specimen having non-threaded holes therethrough on different radii thereof, a plunger secured in each of said non-threaded holes in for movement within said hole, said plunger positioned to contact the end of said specimen and for movement by said rotatable wheels for depressing and relaxing said specimen.
 6. A multispecimen fatigue cracking device as claimed in claim 5; which comprises, a lock nut on said bolts which lock said bolts in place in order to secure one end of said specimen in a depressed position. 